The present invention relates to a method of die casting to form a closed deck engine block.
In the operation of an internal combustion engine (ICE), the cylinders generate waste heat. To draw away this heat, an interconnected continuous cooling passage known as a water jacket is provided around the cylinders and cooling fluid is circulated through the water jacket to a radiator. In manufacturing an ICE, an engine block may be die cast with a permanent reusable steel mold or “die”. The engine block typically forms the cylinder walls—in some cases lined with cylinder liners—and the water jacket around the cylinder walls. The top of the engine block is known as the deck; the crank case is provided at the bottom of the engine block.
In one method of die casting an engine block, steel die core pieces are extended into the die cavity of a permanent steel mold. After die casting, these die core pieces are withdrawn to form a continuous water jacket around the cylinders extending from the deck of the engine block. Later in the manufacturing operations, a cylinder head is joined to the deck of the engine block to cap the cylinders and the water jacket.
With this method of manufacture, after die casting the engine block, the entire water jacket fully opens to the deck. For this reason, this method of manufacture is said to result in an open deck engine block wherein the upper end of the cylinder walls are not connected to the outer walls of the engine block at the deck. Because the engine block is manufactured using a permanent steel mold, melt can be introduced at high pressure and speed into the die cavity allowing an engine block to be formed quickly. Moreover, after forming an engine block, the same die is ready to be reused immediately. Thus, the time to manufacture engine blocks in high volumes is low as compared with other common manufacturing methods. Further, the steel die has a lifespan of many (tens of thousands of) cycles. For all of these reasons, this method of manufacturing allows for the high volume production of engine blocks at a significantly lower cost than that of other common manufacturing methods.
With technological advances and stricter fuel economy standards, there is a need for higher power density (i.e., higher HP/L) ICEs. Higher power density ICEs impart increased pressures and forces to the cylinder walls. This leads to an increase of damaging distortions and vibrations of the freestanding cylinder walls. Therefore, in order to strengthen and support the cylinder walls, manufacturing techniques have been developed to form engine blocks with bridges that extend along the deck between the cylinder walls and the remainder of the engine block. Engine blocks formed with these bridges are known as closed deck engine blocks.
In one approach to manufacturing a closed deck engine block, termed a “lost core” approach, sacrificial cores are positioned within the die cavity of a semi-permanent mold between die core pieces that are extended into the mold. After die casting, the die core pieces are withdrawn leaving the sacrificial cores behind within the engine block casting. These sacrificial cores, which may be formed of sand held together with a binder, can then be broken up and removed from the cast engine block. To facilitate this, the engine block may be formed with holes in its sides that extend to the sacrificial cores. After removal of the sacrificial cores, these holes may be plugged with core plugs. The die core pieces are spaced from one another and the sacrificial cores only intermittently come up to the level of the deck. In consequence, bridges are formed between the cylinder walls and the remainder of the engine block between the die core pieces.
A drawback with this manufacturing approach is that, because the sacrificial cores are relatively fragile, the die casting has to be done at low pressure which slows the cycle time of the die, thereby increasing manufacturing cost. Manufacturing cost is further increased by the fact that the sacrificial cores are lost after each cycle such that they must be replaced after each cycle and due to the additional time needed to properly place the sacrificial cores before casting and break up and remove them after casting.
U.S. Pat. No. 8,820,389 issued Sep. 2, 2014 to Degler describes a lost core manufacturing approach intended to permit high pressure die casting (HPDC). A salt core is molded around a number of cylinder sleeves to create composite cores. The composite cores are then placed within the cavity of a permanent mold. The cylinder sleeve provides support for the salt core to allow HPDC. After casting, the cylinder sleeves of the composite cores are the cylinder walls. The salt cores are then sacrificed (chemically dissolved) to provide the water jacket. As with the low pressure lost core approach, manufacturing costs are increased by the fact the salt cores are lost after each cycle and must be replaced and due to the additional time needed to properly place the sacrificial cores before casting and dissolve and remove them after casting.
A further approach to manufacturing an HPDC closed deck engine block is referred to as the “loose core” approach. With this approach, die core pieces connected to the die are extended into the die cavity so as to surround segments of what will form a cylinder wall and loose die core pieces are inserted between the connected die core pieces. After die casting, the connected die core pieces are withdrawn and the loose die core pieces are knocked out. The loose core pieces form undercuts which interconnect the water jacket around the cylinders and leave bridges between the cylinder walls and the remainder of the engine block along the deck of the engine block. U.S. Pat. No. 6,415,848 issued Jul. 9, 2002 to Komazaki et al. describes an example loose core manufacturing approach.
The loose core manufacturing approach allows HPDC but may leave flash between the loose die core pieces and the connected die core pieces that has to be removed. Further, bridges formed in the loose core approach may need to be fairly narrow in order to facilitate removal of the loose cores. Yet further, the water jacket may need to be enlarged adjacent each deck bridge to provide room to remove the loose core that formed the bridge. Asymmetric water jackets are not preferred due to the less efficient cooling fluid flow patterns through such water jackets. Furthermore, the time to properly locate the loose cores in the die prior to casting and remove them after casting increases manufacturing cost.